P-selectin (CD62, GMP-140, PADGEM), a Ca.sup.2+ -dependent lectin on activated platelets and endothelium, functions as a receptor for myeloid cells by interacting with sialylated, fucosylated lactosaminoglycans. P-selectin binds to a limited number of protease-sensitive sites on myeloid cells, but the protein(s) that carry the glycans recognized by P-selectin are unknown. Blotting of neutrophil or HL-60 cell membrane extracts with .sup.125 I!P-selectin and affinity chromatography of .sup.3 H!glucosamine-labeled HL-60 cell extracts were used to identify P-selectin ligands. A major ligand was identified with an approximately 250,000 M.sub.r under nonreducing conditions and approximately 120,000 under reducing conditions. Binding of P-selectin to the ligand was Ca.sup.2+ dependent and was blocked by mAbs to P-selectin. Brief sialidase digestion of the ligand increased its apparent molecular weight; however, prolonged digestion abolished binding of P-selectin. Peptide:N-glycosidase F treatment reduced the apparent molecular weight of the ligand by approximately 3,000 but did not affect P-selectin binding. Western blot and immunodepletion experiments indicated that the ligand was not lamp-1, lamp-2, or L-selectin, which carry sialyl Le.sup.x, nor was it leukosialin, a heavy sialylated glycoprotein of similar molecular weight. The preferential interaction of the ligand with P-selectin suggests that it may play a role in adhesion of myeloid cells to activated platelets and endothelial cells.
The selectins are three structurally related membrane glycoproteins that participate in leukocyte adhesion to vascular endothelium and platelets, as reviewed by McEver, "Leukocyte interactions mediated by selectins" Thromb. Haemostas. 66:80-87 (1991). P-selectin (CD62), previously known as GMP-140 or PADGEM protein, is a receptor for neutrophils, monocytes and subsets of lymphocytes that is rapidly translocated from secretory granule membranes to the plasma membrane of activated platelets, as reported by Hamburger and McEver, "GMP 140 mediates adhesion of stimulated platelets to neutrophils" Blood 75:550-554 (1990); Larsen et al., "PADGEM protein: a receptor that mediates the interaction of activated platelets with neutrophils and monocytes" Cell 59:305-312 (1989) and endothelial cells, as reported by Geng et al., "Rapid neutrophil adhesion to activated endothelium mediated by GMP-140" Nature 343:757-760 (1990); Lorant et al., "Coexpression of GMP 140 and PAF by endothelium stimulated by histamine or thrombin: A juxtacrine system for adhesion and activation of neutrophils" J. Cell Biol. 115:223-234 (1991).
E-selectin (ELAM-1) is a cytokine-inducible endothelial cell receptor for neutrophils, as reported by Bevilacqua et al., "Identification of an inducible endothelial leukocyte adhesion molecule" Proc. Natl. Acad. Sci. USA 84:9238-9242 (1987), monocytes, as reported by Hession et al., "Endothelial leukocyte adhesion molecule 1: Direct expression cloning and functional interactions" Proc. Natl. Acad. Sci. USA 87:1673-1677 (1990), and memory T cells, as reported by Picker et al., "ELAM-1 is an adhesion molecule for skin homing T cells" Nature (London) 349:796-799 (1991); Shimizu et al., "Activation-independent binding of human memory T cells to adhesion molecule ELAM 1" Nature (London) 349:799-802 (1991). L-selectin (LAM-1, LECAM-1), a protein expressed on myeloid cells and most lymphocytes, participates in neutrophil extravasation into inflammatory sites and homing of lymphocytes to peripheral lymph nodes, as reported by Lasky et al., "Cloning of a lymphocyte homing receptor reveals a lectin domain" Cell 56:1045-1055 (1989); Siegelman et al., "Mouse lymph node homing receptor cDNA clone encodes a glycoprotein revealing tandem interaction domains" Science (Wash.DC) 243:1165-1172 (1989); Kishimoto et al., "Neutrophil Mac-1 and MEL-1 adhesion proteins inversely regulated by chemotactic factors" Science (Wash. DC) 245:1238-1241 (1989); Watson et al., "Neutrophil influx into an inflammatory site inhibited by a soluble homing receptor IgG chimaera" Nature (London) 349:164-167 (1991).
Each selectin functions as a Ca.sup.2+ -dependent lectin by recognition of sialylated glycans. Both E- and P-selectin interact with sialylated, fucosylated lactosaminoglycans on opposing cells, including the sialyl Le.sup.x tetrasaccharide, as reported by Phillips et al., "ELAM 1 mediates cell adhesion by recognition of a carbohydrate ligand, sialy-Le.sup.x " Science (Wash DC) 250:1130-1132 (1990); Walz et al., "Recognition by ELAM-1 of the sialyl-Le.sup.x determinant on myeloid and tumor cells" Science (Wash. DC) 250:1132-1135 (1990); Lowe et al., "ELAM 1 dependent cell adhesion to vascular endothelium determined by a transfected human fucosyltransferase cDNA" Cell 63:475-484 (1990); Tiemeyer et al., "Carbohydrate ligands for endothelial-leukocyte adhesion molecule 1" Proc. Natl. Acad. Sci. USA 88:1138-1142 (1991); Goelz et al., "ELFT: a gene that directs the expression of an ELAM-1 ligand" Cell 63:1349-1356 (1990); Polley et al., "CD62 and endothelial cell-leukocyte adhesion molecule 1 (ELAM 1). recognize the same carbohydrate ligand. sialyl-Lewis x" Proc. Natl. Acad. Sci. USA 88:6224-6228 (1991); Zhou et al., "The selectin GMP-140 binds to sialyated, fucosylated lactosaminoglycans on both myeloid and nonmyeloid cells" J. Cell Biol. 115:557-564 (1991). However, the precise carbohydrate structures on myeloid cells recognized by these two selectins under physiologic conditions are not known. Such ligands might have unique structural features that enhance the binding specificity and/or affinity for their respective receptors.
P-selectin isolated from human platelets binds with apparent high affinity to a limited number of sites on neutrophils (Moore et al., "GMP 140 binds to a glycoprotein receptor on human neutrophils: evidence for a lectin-like interaction" J. Cell Biol. 112:491-499 (1991); Skinner et al., "GMP-140 binding to neutrophils is inhibited by sulfated glycans" J. Biol. Chem. 266:5371-5374 (1991) and HL-60 cells (Zhou et al., "The selectin GMP-140 binds to sialyated, fucosylated lactosaminoglycans on both myeloid and nonmyeloid cells" J. Cell Biol. 115:557-564 (1991)). Binding is abolished by treatment of the cells with proteases (Moore et al., 1991), suggesting that the glycans on myeloid cells recognized preferentially by P-selectin are on glycoprotein(s) rather than on glycolipids. The number of binding sites for platelet P-selectin on neutrophils has been estimated at 10,000-20,000 per cell (Moore et al., 1991; Skinner et al., 1991), suggesting that these sites constitute a small component of the total cell surface protein. The protein portion of this ligand(s) may be crucial for binding by presenting the glycan in an optimal configuration, clustering glycans to enhance avidity, favoring the formation of specific oligosaccharide structures by cellular glycosyltransferases or modifying enzymes, and/or stabilizing the lectin-carbohydrate interaction through protein-protein interactions with P-selectin.
The potential importance of protein components in enhancing ligand affinity is supported by studies of CHO cells transfected with a specific fucosyltransferase (Zhou et al., 1991). These cells express higher amounts of the sialyl Le.sup.x antigen than do HL-60 cells and have protease-sensitive binding sites for P-selectin. However, the interaction of P-selectin with these sites is of much lower apparent affinity than with those on myeloid cells, and adhesion of transfected CHO cells to immobilized P-selectin is weaker than that of neutrophils and HL-60 cells (Zhou et al., 1991). These observations suggest that myeloid cells express one or more membrane glycoproteins not found on CHO cells that enhance the lectin-mediated interaction with P-selectin. Alternatively, myeloid cells may express a glycosyltransferase or modifying enzyme not present in CHO cells.
It is therefore an object of the present invention to identify and characterize a specific glycoprotein ligand for P-selectin (CD62).
It is a further object of the present invention to provide methods and compositions derived from the characterization of a specific glycoprotein ligand for P-selectin for use in modifying inflammatory processes and in diagnostic assays.